Many methods have been published in the scientific and technical literature describing means for achieving improvement in the adhesive bond between elastomers and various substrates. While this effort has led to many patents and some significant improvements in this adhesive bond, there is still a considerable need for better bonding techniques. A particularly fruitful modification of natural rubber and certain synthetic elastomers has been incorporation of carboxylic acid functionality into the rubber chain through copolymerization or grafting techniques with such monomers as acrylic and methacrylic acid or the esters of these acids, which may later be converted to carboxylic acid functionality by saponification of the esters. These techniques have resulted in useful elastomers with significant bonding capabilities, but these methods have not received general acceptance for the reason that the elastomers formed by such techniques are usually viscous and hard to handle, the polymerization chemistry is complicated and difficult, and the products are not compatible with a wide range of other elastomers. To a limited extent, maleic acid, fumaric acid, itaconic acid and the anhydride derivatives of maleic acid and itaconic acid have been used to graft or adduct solid elastomers with carboxylic acid functionality (Trivedi, B.C. Culbertson, B.M. (1982) "Maleic Anhydride," Plenum Press). Again these methods suffer from mechanical difficulties associated with handling the high molecular weight solid elastomer during the chemical reaction sequences.
A more desirable possibility from the standpoint of the rubber compounder who needs the flexibility to compound almost any elastomer specifically for an adhesive requirement would be the addition of lower molecular weight carboxylic acids or derivatives to the rubber compound during mixing of the rubber compound. While in some special applications this has been achieved, this method has not received general acceptance because most carboxylic acids and derivatives do not behave well in the mixing step due to high melting points or low solubility in rubber compounds. These materials are not usually compatible with the finished unvulcanized elastomer, and indeed, also tend to interfere with the vulcanization step.
In part, the difficulty with adhesion of elastomers to a variety of substrates lies in the generally non-polar nature of most natural and synthetic elastomers which do not contain bonding species which can react or coordinate with the generally polar bonds at the interface with a mineral, fiber or metal surface. In addition, those surfaces that do not contain polar bonding surfaces are inert to most kinds of reactions which might provide adhesive interface with non-polar bonds. There are physical problems also in that the adhesive interface for this type of bond rarely contains bonding elements having the same coefficient of expansion, or the same elastic modulus, for example.
Methods to bring about satisfactory bonds have involved the following types of transformations:
a. Chemical methods to modify the elastomer interface by chemically altering the surface of the elastomer with polar bonds. This approach is exemplified by chlorination of the elastomer surface with sodium hypochlorite solutions or other forms of chlorine containing treatments as described by Schidrowitz, P. and Redfarn, C.A. (1935) J. Soc. Chem. Ind. 54:263T. Another approach is exemplified by treatment of the surface of polytetrafluoroethylene with sodium napthalide which is said to abstract fluorine from the surface leaving acetylide linkages which are more compatible surfaces for metal bonding. (Nelson, E.R. et al. (1958) Ind. Eng. Chem. 50:329.) PA1 b. Chemical methods to modify the substrate interface by chemically altering the surface of the substrate with non-polar bonds. This approach is exemplified by treating the metal surface with primer systems designed to impart a bonding surface which is more compatible with non-polar elastomers. There are many commercial primer systems which are applied to a wide variety of bonding applications which use this technique. For example, see literature describing Lord Elastomer Products ChemLok.RTM. 205 Rubber-to-Metal Adhesive Primer. PA1 c. Physically modifying the substrate interface with a coating which bonds to both elastomeric and substrate surfaces with greater bonding energy than either alone can be made to bond. For example, sputter coated brass on steel wire, bonded with sulfur-cured rubber (Von Ooij, W.J. (1979), "Fundamental Aspects of Rubber Adhesion to Brass-Plated Steel Tire Cords," Rubber Chem. Technol. 52:605-675). PA1 d. The combination of several of these techniques at once. Because bonding various surfaces is fraught with technical difficulties, combinations of methods is frequently used in commercial systems. These techniques may involve as many as seven treatment steps. For example, directions for the use of Lord Elastomer Products ChemLok.RTM. 252 describes these steps. PA1 a) adhesion of elastomers to other elastomers; PA1 b) adhesion of elastomers to plastic materials; PA1 c) adhesion of elastomers to metal substrates, e.g., adhesion of natural, polybutadiene or styrene butadiene rubber to brass-coated steel wire; PA1 d) adhesion of elastomers to fabrics, fibers, paper and miscellaneous substrates; and PA1 e) adhesion of elastomers to glass, mineral fillers and coarse mineral substrates. PA1 (a) reacting an unsaturated polymer as described above with a dicarboxylic acid or derivative to form an unsaturated polymeric dicarboxylic acid adduct wherein said acid or derivative moiety comprises at least about three weight percent of said adduct; and PA1 (b) adding said adduct to an uncured elastomer in an amount between about 2 and about 50 weight percent of said uncured adhesive elastomeric composition.
The various bonding techniques described briefly above have been applied to these bonding problems in a bewildering array of methods, but in general have failed to provide either a truly satisfactory or a generally useful method. Several successful approaches have used some of the chemical reactions which occur during vulcanization to bring about a corresponding or similar reaction at the bonding surface. An example is the use of sulfur and sulfur donors Which are principally added to participate in the vulcanization reactions to also participate with metal oxide or metal sulfide bonds on the substrate surface. This technique is described repeatedly in the scientific literature, for example in elastomer to brass coated wire adhesion. See, e.g., van Ooij, J. (1984) Rubber Chem. Technol. 57:421-456.
A milling process has been described, where chemical bonds have been broken between rubber fragments by mechanical shear forces which have reformed during milling in the presence of maleic anhydride to give a maleinized rubber. Such processes have found useful applications as described in Bacon, R.G.R. and Farmer, E.H. (1939) Rubber Chem. Technol. 12:200-209. An anhydride adducted rubber was described in 1944 for bonding natural rubber to artificial silk. (Kambara, S. et al. (1944) Soc. Chim. Ind. Japan 45:141-143; CA. 43, 1595 (1949)). The problem with this approach is that it is very difficult to obtain higher concentrations of chemically bound maleic anhydride in the rubber compound, the mechanical mixers used to obtain high mixing energy do not contain the odorous and toxic vapors of maleic anhydride in a satisfactory way, thus causing unsafe working conditions around the mixer, and the mixing energy needed to mechanically break rubber chains to bring about this reaction is unfortunately great.
Maleic adducts have long been known to the coatings industry where such natural oils as linseed and soya oil have been successfully maleinized and used commercially for many years. These natural drying oils have limited unsaturation content however, such that such vegetable oils cannot be highly maleinized. Most of these oils contain a maximum of three unsaturation sites which can be maleinized. Maleinized vegetable oils have been used as additives to rubber compositions, but the results have not been generally applicable or remarkable to adhesive bonds. Partly, these materials have limited solubility in rubber compounds, and this appears to limit their usefulness. These maleinized vegetable oils have proven useful in coatings, where the presence of the maleinized products have shown good film adhesion and have exhibited other useful coating properties.
Low molecular weight polybutadienes and other highly unsaturated polymers have been maleinized and used as chemical intermediates for the production of air dried coatings and electrodepositional primer coatings, and these products have been very useful for these purposes. Good adhesion properties are typically observed with these coatings. Medium molecular weight resins of this type have not been much in demand for the coatings industry partly due to the high viscosity of these systems, and have thus not been readily available. It is precisely this medium molecular weight range however which is most useful in the context of this invention.
A maleinized polyisoprene resin is described in U.S. Pat. No. 4,218,349 assigned to Kuraray, Ltd of Japan and is used in a sulfur cured natural rubber blend to provide improved green strength of the natural rubber compounds and, incidentally, to provide improved adhesion to metal. No mention of the adhesive property of such compounds with other elastomers and with other cure systems is given, except for blends with natural rubber comprising less than 35% of total rubber content of synthetic elastomer of greater than 300,000 molecular weight. A similar material is described in U.S. Pat. No. 4,204,046 also assigned to Kuraray, Ltd of Japan for use as a constituent of a pressure sensitive adhesive, but no description is given of this resin compounded with elastomers of any kind (compounds referred to as tackifiers, which are not considered to be elastomers, are described).
Kuraray Pat. No. 4,218,439 teaches that the physical properties and processability of the vulcanizate are decreased with excessive amounts of maleic anhydride. Applicants have observed that adhesion can be doubled or better over that of the Kuraray patent and that the more dicarboxylic acid (e.g., maleic acid) moiety present, the better the adhesion. In light of the Kuraray teachings it was surprising to find that both natural and synthetic rubber compositions could be prepared which retained physical properties and processability.
None of the foregoing disclosures teach an adhesive elastomeric composition comprising a polymeric dicarboxylic acid adduct wherein the polymer has a cis-1,4 content less than 70%, or wherein the polymer has a molecular weight less than 8,000. Moreover, none of the foregoing disclosures teach an adhesive elastomeric composition using synthetic rubber and not containing natural rubber, which comprises a dicarboxylic acid adduct.